1. Field of the Invention
The present invention relates to a color thermal printer having a tension roller. More particularly, the present invention relates to a color thermal printer which has a tension roller, and in which transport of recording sheet can be equalized between an advance direction and a return direction, to raise a speed of printing.
2. Description Related to the Prior Art
A color thermal printer is known, in which a roll of continuous color thermosensitive recording sheet is used. A thermal head is used to record an image with pressure and heat to the continuous recording sheet being transported. Images of three primary colors are recorded in a frame-sequential recording, to obtain a full-color print. To shorten the printing time in the color thermal printer, the continuous recording sheet can be transported back and forth. A first one of the colors is recorded in the transport in the advance direction. A second one of the colors is recorded in the transport in the return direction. A third one of the colors is recorded in the transport in the forward direction for a second time.
To transport the continuous recording sheet back and forth with respect to the thermal head, there is a known structure in which a platen roller is opposed to the thermal head, disposed to receive a back surface of the continuous recording sheet, and caused to rotate for transporting the continuous recording sheet. However, the platen roller is produced from rubber, and is difficult to transport the continuous recording sheet with high precision due to influences of micro slips or the like. Shifts in registration of the colors may occur. Therefore, at least one feed roller set including a pair of two rollers is used for transporting the continuous recording sheet. In the feed roller set, there are a capstan roller and a pinch roller for being pressed against the continuous recording sheet to squeeze the continuous recording sheet between the same and the capstan roller. Furthermore, two feed roller sets may be used, and disposed in positions upstream and downstream from the thermal head. A first one of the feed roller sets operates to transport the continuous recording sheet in the advance direction. A second one of the feed roller sets operates to transport the continuous recording sheet in the return direction.
If the feed roller set for transport in the advance direction is different from the feed roller set for transport in the return direction, it is difficult to transport the continuous recording sheet at an equal amount between the two directions. This causes a problem of shifts in the registration of the colors.
Furthermore, the single structure of the feed roller set may be used for the continuous recording sheet in the advance and return directions. However, tension applied by the feed roller set to the continuous recording sheet changes between the two directions. So unwanted changes occur in the transporting amount of the continuous recording sheet.
In view of the foregoing problems, an object of the present invention is to provide a color thermal printer which has a tension roller, and in which transport of recording sheet can be equalized between an advance direction and a return direction, to raise a speed of printing.
In order to achieve the above and other objects and advantages of this invention, a color thermal printer for recording at least three color images to thermosensitive recording material according to frame sequential recording is provided. A feed roller set transports the recording material at a first speed back and forth by transport in first and second directions alternately. A thermal head is positioned downstream from the feed roller set in the first direction, for recording the three color images according to the transport in both the first and second directions. A first tension roller set is positioned downstream from the thermal head in the first direction, for applying tension to the recording material. A first tension motor drives the first tension roller set. A controller controls a rotational speed of the first tension motor, determines a transporting speed of the recording material with the first tension roller set higher than the first speed in the transport in the first direction, and determines a transporting speed of the recording material with the first tension roller set lower than the first speed in the transport in the second direction. A first torque limiter adjusts transmission of rotation of the first tension motor to the first tension roller set, to keep a transporting speed of the recording material with the first tension roller set at the first speed during the transport in the first and second directions.
To be precise, a color thermal printer has a thermal head for thermal recording to recording material. There is a feeding motor. At least one feed roller is positioned upstream from the thermal head as viewed in an advance direction, caused by the feeding motor to rotate forwards, for transporting the recording material in the advance direction by applying a predetermined transporting force thereto, and also caused by the feeding motor to rotate backwards, for transporting the recording material in the return direction by applying the predetermined transporting force thereto. At least one first tension roller is positioned downstream from the thermal head as viewed in the advance direction, for frictionally contacting the recording material. A first tension motor rotates the first tension roller forwards and backwards. A controller drives the feeding motor in a forward direction, and simultaneously causes the first tension motor to rotate the first tension roller forwards, to apply a first transporting force to the recording material, the first transporting force being higher than the predetermined transporting force, the controller drives the feeding motor in a backward direction, and simultaneously causes the first tension motor to rotate the first tension roller backwards, to apply a second transporting force to the recording material, the second transporting force being lower than the predetermined transporting force. A torque limiter is secured between an output shaft of the first tension motor and the first tension roller in a slippable manner, for applying load to one of the output shaft and the first tension roller relative to a remaining one thereof, to keep the first tension roller rotated at a rotational speed adjusted according to the predetermined transporting force simultaneously with application of the first or second transporting force to the recording material, transport of the recording material being stabilized according to a difference between the predetermined transporting force and the first or second transporting force.
In a preferred embodiment, the at least one first tension roller comprises two rollers for nipping the recording material.
The feeding motor is a stepping motor, the predetermined transporting force is predetermined according to a period of the thermal recording of the thermal head by one line. The first tension motor is a DC motor.
The thermal head records at least first, second and third colors to the recording material in frame-sequential recording, transport of the recording material in the advance direction is used for recording the first color, and transport of the recording material in the return direction is used for recording the second color.
The predetermined transporting force is higher than load applied by the thermal head to the recording material in the thermal recording. The torque limiter is operated in the thermal recording to provide the load.
Furthermore, at least one second tension roller is positioned upstream from the feed roller as viewed in the advance direction, for frictionally contacting the recording material. A second tension motor rotates the second tension roller forwards. Simultaneously with driving of the feeding motor in the forward direction, the controller further causes the second tension motor to rotate the second tension roller forwards, to apply a third transporting force to the recording material, the third transporting force being lower than the predetermined transporting force. The first and third transporting forces are so predetermined as to provide equilibrium of a difference between the first and predetermined transporting forces to a difference between the third and predetermined transporting forces.
The first tension roller includes a roller body. A cylindrical holder chamber is formed in the roller body. A transmission shaft is inserted in the holder chamber, for being rotated by the first tension motor. The torque limiter applies the load to one of the transmission shaft and the roller body relative to a remaining one thereof.
The torque limiter includes a housing, disposed in the holder chamber, and having a ring chamber inside. A first retention mechanism retains the housing in the holder chamber. An inner ring is disposed in the ring chamber, for receiving insertion of the transmission shaft. A second retention mechanism retains the inner ring on the transmission shaft. A coil spring applies the load frictionally to one of the housing and the inner ring relative to a remaining one thereof.
The first retention mechanism includes a groove formed in an inner wall of the holder chamber. A projection is formed to project from the housing, for engagement with the groove.
The second retention mechanism includes a pin formed to project from the transmission shaft. A notch is formed in the inner ring, for engagement with the pin.
According to another preferred embodiment, a feed roller set transports the recording material at a first speed back and forth by transport in first and second directions alternately. A thermal head is positioned downstream from the feed roller set in the first direction, for recording the three color images according to the transport in both the first and second directions. A platen roller supports the recording material pressed by the thermal head. A platen motor drives the platen roller. A controller controls a rotational speed of the platen motor, determines a transporting speed of the recording material with the platen roller higher than the first speed in the transport in the first direction, and determines a transporting speed of the recording material with the platen roller lower than the first speed in the transport in the second direction. A first torque limiter adjusts transmission of rotation of the platen motor to the platen roller, to keep a transporting speed of the recording material with the platen roller at the first speed during the transport in the first and second directions.
According to still another preferred embodiment, a feed roller set transports the recording material at a first speed back and forth by transport in first and second directions alternately. A thermal head is positioned downstream from the feed roller set in the first direction, for recording the three color images according to the transport in both the first and second directions. A first tension roller set is positioned downstream from the thermal head in the first direction, for applying tension to the recording material. A first tension motor drives the first tension roller set, a rotational speed of the first tension motor being changeable with an increase or decrease according to a transporting speed of the recording material. A second tension roller set is positioned upstream from the feed roller set in the first direction, for applying tension to the recording material. A second tension motor drives the second tension roller set, a rotational speed of the second tension motor being changeable with an increase or decrease according to a transporting speed of the recording material. A controller controls the rotational speeds of the first and second tension motors. In a first process of the transport in the first direction, the controller adjusts the rotational speed of the first tension motor to determine a transporting speed of the recording material with the first tension roller set higher than the first speed, and adjusts the rotational speed of the second tension motor to determine a transporting speed of the recording material with the second tension roller set lower than the first speed, the first tension roller set creates forward tension to pull the recording material in the first direction, the second tension roller set creates back tension to pull the recording material in the second direction, the back tension is substantially equal to the forward tension. In a second process of the transport in the second direction, the controller adjusts the rotational speed of the first tension motor to determine a transporting speed of the recording material with the first tension roller set lower than the first speed, and adjusts the rotational speed of the second tension motor to determine a transporting speed of the recording material with the second tension roller set higher than the first speed, the second tension roller set creates forward tension to pull the recording material in the second direction, the first tension roller set creates back tension to pull the recording material in the first direction, the back tension is substantially equal to the forward tension.
To be precise, a color thermal printer includes a thermal head for thermal recording to recording material. There is a feeding motor. At least one feed roller is positioned upstream from the thermal head as viewed in an advance direction, caused by the feeding motor to rotate forwards, for transporting the recording material in the advance direction by applying a predetermined transporting force thereto. At least one first tension roller is positioned downstream from the thermal head as viewed in the advance direction, for frictionally contacting the recording Material. At least one second tension roller is positioned upstream from the feed roller as viewed in the advance direction, for frictionally contacting the recording material. First and second tension motors rotate respectively the first and second tension rollers forwards. A controller drives the feeding motor in a forward direction, and simultaneously rotates the first tension roller forwards by providing the first tension motor with first energy, to apply first transporting force to the recording material, the first transporting force being higher than the predetermined transporting force, and also simultaneously rotates the second tension roller forwards by providing the second tension motor with second energy, to apply second transporting force to the recording material, the second transporting force being lower than the predetermined transporting force. The first and second energies are so predetermined as to provide equilibrium of a difference between the first and predetermined transporting forces to a difference between the second and predetermined transporting forces.